The production of chlorine by electrolysis of alkali chloride solutions, with particular reference to sodium chloride and potassium chloride (hereinafter “brine”) is currently carried out according to three different processes, namely the ion-exchange membrane process, the porous diaphragm process, and the mercury cathode process. The latter type, based on a long-known technology, has experienced a continuous improvement in the cell structure (Ullmann's Encyclopaedia of Industrial Chemistry, VCH, Vol. A6, pag. 416) essentially directed to decreasing the electric energy consumption and to preventing the release of mercury into the environment.
The problem of energy consumption reduction was tackled with success by replacing the original graphite anodes with titanium anodes activated with a particularly effective coating based on platinum group metal oxides. The activated titanium anodes are also characterised by a long operative lifetime, allowing a substantial reduction in the amount of cell shut-downs, which were quite frequent in the case of the corrodible graphite anodes. Since the maintenance shut-downs are critical as regards the release of mercury into the environment, the benefit obtained under this standpoint is apparent.
A further mercury leak reduction was obtained by the routine use of recrystallised salt which permits minimising the quantity of mercury-polluted muds purged from the brine purification section, although involving an additional cost. As a consequence of these provisions it can be nowadays demonstrated that the mercury release from a well-designed and correctly handled plant does not exceed 3 grammes per tonne of product chlorine versus 10 grammes of about ten years ago (Ullmann's Encyclopaedia of Industrial Chemistry, VCH, Vol. A6, page 424).
In currently operating plants, the cathodic caustic product, normally consisting of caustic soda or potash, exiting the amalgam decomposers and containing significant amounts of mercury, graphite powder and hydrogen, is flowed through drippers consisting of perforated plates which cause its fractioning into droplets, with the purpose of breaking the electrical continuity thereby eliminating or at least substantially reducing the stray currents, which consist of parasitic electric current discharging some of the cell voltage to the ground. Stray currents have a negative effect since they lessen the overall electrical efficiency of the process, and more importantly because they determine corrosive attacks which can be very severe.
Since the caustic product contains significant amounts of mercury dragged in the form of microdroplets, the soda or potash discharged from the drippers, prior to being sent to storage, is made to flow through filters containing active carbon, capable of absorbing the mercury present therein and reducing its outlet concentration to very low values, usually compatible with the user specifications. Such treatment, whose results in terms of product final quality are certainly positive, presents the inconvenience of requiring a frequent renovation of the active carbon bed, which is rather quickly saturated with mercury. This operation, inevitably entailing a manual intervention, is hazardous for the health of the operators and produces remarkable quantities of spent carbons that have to be disposed of at high costs.
It would be desirable to overcome the inconveniences associated with the methods of cathodic caustic product treatment currently employed in the presently operating chlor-alkali plants.